Photon-emission microscopy is commonly used to localize failures in electronic integrated circuits. The technique detects light emitted from defects and failed elements when a device under test (DUT) is biased. Because the resolution of photon-emission microscopy is at best about 1 micron, localization of a defect is not usually sufficient to determine the cause of the failure or which element of a device has failed. Spectral analysis of the emitted light is of interest because it can provide additional information on the failure cause and place.
A simple method of carrying out a spectral analysis of the emitted fight is to use a series of narrow bandpass filters--each filter being used to examine a narrow wavelength interval of the spectrum. Such systems are described in U.S. Pat. No. 5,006,717 and U.S. Pat. No. 5,320,830. However, a disadvantage with filters is their relatively low transmittance and the resulting low light levels and long acquisition times for capturing the spectrum during which the DUT is biased and may deteriorate further. In addition filters only provide spectral analysis at discrete wavelengths.
An improved technique which is capable of analyzing continuous spectra is described in U.S. Pat. No. 5,569,920 and U.S. Pat. No. 5,724,131. The known method uses a first detector for directly viewing the DUT and for localization of an emission. For spectral analysis, a high efficiency semi-ellipsoid mirror is placed directly above the DUT to collect the emitted light. The light collected by the mirror is coupled through an optical fiber to a grating monochromator connected to a photomultiplier. This system requires two detectors and in the case where more than one emission spot is present on the DUT all the light from the emission spots will be collected and overlapping of their spectra cannot be avoided. Further, placing any item such as the mirror close to the DUT limits the types of DUT which can be examined. For example, if the DUT is an integrated circuit on a wafer (not yet packaged) the mirror may mechanically interfere with the needle probes required to feed power to the DUT. Further, placing the mirror between the microscope objective lenses and the DUT places a limitation on the objective lenses and how close they can work to the DUT.
Another microscope system is known from the article by Kees de Kort and Paul Damink, "The spectroscopic signature of light emitted by integrated circuits", Proc. ESREF, pp. 45-52, 1990 and makes use of a beam splitter, a prism and a focusing lens. This method also requires two detectors which is a disadvantage, one for normal viewing and a second one for detecting the light split by the beam splitter towards the monochromator prism and lens. Also, the already weak light emitted from the biased DUT is partially lost in the beam splitter.